1. Field of Invention
The present invention relates generally to the field of software defined radio (SDR) in mixed-signal communication channels.
2. Description of Related Art
A modem is a device for modulating and demodulating a signal that typically has digital information therein, hence the term “modem” for modulator/demodulator. The signal is constructed to be suitable for the relevant transmission medium and is typically transmitted as an analog signal.
Modems have been used to communicate via telephone lines, with an analog carrier signal encoded with digital information, but modems can be used over any medium for transmitting signals, including over-the-air radio waves. Radio waves are generally defined to include the band of radio frequencies in the electromagnetic spectrum from 3 kHz to 300 GHz, from the Extremely Low Frequency (ELF) to Extremely High Frequency (EHF) bands respectively.
In this application, modem is used as a term for “modem control”, meaning managed resources for waveform modulation and demodulation schemes of the kind used in radio data transmission, including but not limited to Frequency Modulation (FM), Amplitude Modulation (AM), Single Side Band (SSB), Double Side Band (DSB), Vestigial Sideband (VSB), Frequency Shift Keying (FSK), Phase Shift Keying (PSK), Gaussian Minimum Shift Keying (GMSK), Quadrature Amplitude Modulation (QAM), Frequency Hopped Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), Orthogonal Frequency Division Multiplexing (OFDM) and the like.
A software defined radio is a radio that functions like a computer, where the functionality of the radio is defined by software that can be upgraded, rather than by fixed hardware. SDR has been defined as a radio whose signal processing functionality is defined in software; where the waveforms are generated as sampled digital signals, converted from digital to analog via a high-speed Digital-to-Analog Converter (D/A) and then translated to Radio Frequency (RF) for wireless propagation to a receiver. The receiver typically employs an RF subsystem coupled to a high-speed Analog-to-Digital Converter (A/D) that can capture some or all of the channels of the software radio node. The receiver then extracts and demodulates the channel waveform using software executing on a digital processor.
SDR is aimed at solving several of the challenges of over-the-air communications, including compatibility with pre-existing legacy radio systems, ability to emulate transmission and reception of a plurality of different waveforms or forms of modulation (modem control), and more efficient spectrum usage, including operation in different frequency bands, with the lowest possibility of interception, detection and interference from unauthorized parties. One of the first SDRs was the SPEAKeasy SDR, known per se in the art. The GNU Software Radio project (www.gnu.org/software/gnuradio/gnuradio.html) is another well documented SDR initiative.
The US military through the Department of Defense (DoD) has driven the development of next generation SDR with an Open Standard Architecture standard for implementing Joint Tactical Radio Systems (JTRS), which is used to communicate in military communication systems, through the use of an open standard Software Communications Architecture (SCA). The SCA calls out the following features: a Common Open Architecture; the ability to support multiple domains, including airborne, fixed, maritime, vehicular, dismounted and handheld applications; the ability to operate in multiple frequency bands; compatibility with legacy radio systems; the ability to easily insert new technologies to improve performance; enhanced security, including cryptographic capability, user identification and authentication, encryption key management, and multiple independent levels of security classification; networking ability, including support for legacy network protocols; software reusability; and support for plug-and-play and real-time reconfigurability, with waveforms being portable from one implementation to another.
The SCA provides standardization of hardware platforms and waveform application software to enable portability and interoperability over the life cycle of military communication systems. Implementing the SCA, however, introduces overhead that can require substantial additional computing capacity. Current JTRS implementations have used very high speed Commercial-Off-The-Shelf (COTS) microprocessors to achieve the necessary processing capacity at an acceptable price. Small form factor and power constrained applications often cannot tolerate the power required or heat generated by these implementations. Further, military systems need enhanced levels of security capability that may not be possible with COTS components. Still further, military systems need to be scalable to enable a common architectural approach for hosting either relatively small, simple waveforms or large complex waveforms. Prior art scalable architectures typically employ multiple distributed processors communicating with each other over an interconnecting network. An example prior art interconnecting network is the European Space Agency SpaceWire network standard that is described at http://www.estec.esa.nl/tech/spacewire/overview/.
What is needed is a new architectural approach for an SDR that is scalable, low power, high speed, SCA compliant and capable of enhanced security. Thus, the present invention provides a superior method and apparatus for an SDR that is DoD SCA compliant, of the kind used in JTRS systems.